Amplifier systems capable of providing variable output power may include linear amplifiers. An amplifier system which uses conventional linear amplification may produce a highly stable low distortion power output. The low distortion output can make linear amplifiers particularly useful in amplifier system applications that require high accuracy such as, for example, very precise linear motors used in semiconductor photolithography stages.
FIG. 1 shows an amplifier system 101 which includes a conventional linear amplifier 103 which supplies an output signal 113, which may be proportional to an input signal 115, to a variable load (not shown). A positive DC power source 105 and a negative DC power source 107 drive the linear amplifier 103. A control system (not shown) may typically be used to make feedforward adjustments to vary the output signal 113. The variable output signal 113 of amplifier system 101 may be used as a power supply to drive a variable load.
The energy efficiency of amplifier system 101 may be inversely proportional to the differential voltage between the output signal 113 and amplifier power sources 105 and 107. When the linear amplifier 103 is operating at a low differential voltage, the amplifier system 101 may be operating efficiently and much of the energy from power sources 105 and 107 may be converted into output signal 113. Conversely, under high differential voltage operation, the amplifier system 101 may operate inefficiently and a substantial portion of the energy from power sources 105 and 107 may be converted into heat rather than output signal 113.
The power sources 105 and 109 typically supply the linear amplifier 103 with fixed voltage power and the amplifier system 101 output voltage may typically vary proportionally to the output signal 113. When the output signal 113 is high powered, the output voltage may also be high. Conversely, when the output signal 113 is low powered the output voltage may be low. Thus, the amplifier system 101 efficiency may vary with the output power. A higher power output signal 113 may result in low differential voltage and improved efficiency. A low power output signal 113 may result in a high differential voltage and decreased efficiency.
Under variable power operation the voltage of output signal 113 may vary significantly. When amplifier system 101 is operating at the maximum output power, maximum efficiency may be utilized. However, under variable power operation, the output signal 113 voltage may typically not be at the maximum level and amplifier system 101 may generally not be operating at maximum efficiency. Although linear amplifier 103 may be suitable for driving high accuracy applications, under variable loads amplifier system 101 with linear amplifier 103 may not be energy efficient.
The aforementioned problems associated with amplifier systems using conventional linear amplification under variable output load conditions may result in severe operational inefficiency and heat generation. In applications that require precise temperature control, heat dissipation can pose a significant design concern. Amplifier systems using conventional linear amplification may require a complicated enclosure to dissipate the excess heat generated when operating at a high differential voltage. Enclosures capable of dissipating the excess heat may require large capacity cooling mechanisms or other environmental controls, adding complexity and costs to the amplifier system.
As those skilled in the art will appreciate, amplifier systems using pulse width modulated ("PWM") amplification can produce a wide range of output power with more energy efficiency than amplifier systems using conventional linear amplification. Because amplifier systems using PWM amplification have higher energy efficiency under variable output power operation, they may be less expensive to operate than amplifier systems using conventional linear amplification. Further, because amplifier systems using PWM amplification may generate little heat energy, less elaborate cooling systems may be required and the amplifier system may be less expensive to build.
FIG. 2 shows a power supply 201 which includes a conventional PWM amplifier 203 which supplies an output signal 213 to a variable load (not shown), which may be proportional to an input signal 215. A positive DC power source 205 and a negative DC power source 207 may be used to drive the PWM amplifier 203. A control system (not shown) may typically be used to make feed forward adjustments to the output signal 213. The variable output signal 213 of PWM amplifier 203 may be used as a power supply to drive a variable load.
Although amplifier systems which use PWM amplification may produce the same power output as amplifier systems which use linear amplification, PWM amplification may lack the performance characteristics required for some applications. More specifically, an amplifier system using conventional PWM amplification may produce output power which includes a high frequency noise. Amplifier systems using PWM amplification may thus not be suitable for photolithography motor drives and other high performance applications which may require noise free power.
Hence, there is a need for an amplifier system which has the PWM amplification characteristic of high operating efficiency across a wide output power range and the linear amplification characteristic of low noise for compatibility with high performance applications.